The present invention relates to an angular momentum mass flowmeter (AMMF) and more particularly to a fluid-driven AMMF.
An angular momentum mass flowmeter (AMMF) employs a motor- or fluid-driven device to impart a known angular velocity (and hence angular momentum) to the fluid flow to be measured relative to a rotating spring-restrained impeller. As the fluid flow impinges on the vanes of the impeller, the relative angular momentum of the fluid flow produces a torque on the impeller which is proportional to the mass flowrate of the fluid and the angular speed of the impeller. This relative angular momentum sensed by the impeller is interpreted as an indication of the mass flow rate of the fluid.
In some of the AMMF's, the angular momentum is created by an electrically powered device which introduces an angular fluid-flow component to the fluid flow, thereby instilling angular momentum. The modern AMMF's more frequently utilize the momentum of the incoming fluid flow to mechanically introduce an angular fluid-flow component thereto. Since the fluid-driven device does not require an electrical power source, it is generally preferred over the electric motor-driven device because of its lower cost and lower weight. The present invention is directed to an AMMF utilizing a fluid-driven device to generate the angular momentum.
More particularly, in a fluid-driven device the angular momentum is developed by the combined functions of a flow control valve and a swirl generator (or swirl cap) downstream thereof. The swirl generator defines a series of helical grooves about its periphery in order to give the fluid stream passing thereby a swirl velocity (i.e., an angular momentum) as the fluid passes through the helical grooves. The control valve defines a plurality of spring fingers which restrain the fluid flow so that, at the lower flow rates, all of the fluid flow passes through the helical grooves of the swirl generator. As the fluid-flow rate is increased, however, the pressure drop through the helical grooves increases, thereby creating an outward force on the spring fingers. At some point (approximately 1,000 pounds per hour of fluid flow), the outward force developed on the spring fingers is sufficient to lift the spring fingers off and away from the grooved surface of the swirl generator so that not all of the fluid flow enters the helical grooves. As flow is increased beyond this point, the plurality of spring fingers continue to open, thereby allowing an increasing portion of the fluid flow to bypass the helical grooves of the swirl generator. Thus the plurality of spring fingers essentially constitute a pressure-operated valve which uses the pressure drop across the swirl generator to regulate the amount of swirl imparted by the swirl generator to the fluid stream.
The position of the swirl generator relative to the spring fingers and the tension of the spring fingers are among the variables which may be controlled in order to ultimately control speed and "start-up" rates. The position of the spring fingers relative to the swirl generator provides a controlled angular speed of the fluid flow which acts on the downstream turbine vanes and minimizes the level of pressure drop in the fluid-drive section of the flowmeter at the higher fluid-flow rates. In other words, the spring fingers act as a control valve to modify the flow area as a function of pressure drop in order to regulate the angular momentum of the fluid flow and the rate of rotation of the impeller.
The conventional AMMF produces the angular momentum in the fluid flow upstream of the torque-sensing element (i.e., the impeller). The drawback of the conventional AMMF which produces the angular momentum upstream of the torque-sensing element is that the changing flow-passage geometry (which is a function of the spacing between the spring fingers of the control valve and the swirl generator) alters the exit-flow profile from the swirl generator as a function of flow rate. The exit-flow profile can be described as having an average radius of gyration (r) about which fluid flows at a nominal rate and enters the passage of the impeller. The mass flow rate is directly related to the square of the average radius of gyration (r.sup.2) of the flow exiting the impeller. As a result, the accuracy of the flowmeter is dependent upon the stability of the flow-velocity profile of flow entering the impeller. Where the fluid-driven device is disposed upstream of the impeller, changes in the velocity profile of fluid exiting the fluid-driven device influence the impeller located downstream thereof.
Accordingly, it is an object of the present invention to provide an AMMF wherein the fluid-drive device (that is, the swirl generator) is disposed downstream of the torque-sensing element (that is, the impeller).
Another object is to provide such a flowmeter wherein the variable flow profile associated with the fluid stream exiting the helical grooves of the swirl generator over the wide range of possible flow rates does not negatively affect the radius of gyration of the fluid flow through the impeller.
A further object is to provide such a flowmeter wherein the torsion spring is positioned for easy accessibility for repair or replacement, and the skew vane and a sensing coil are easily accessible for calibration purposes.